I'm completely ignorant about the thread, but I can't understand where's the problem.
It only takes to contact Valve, explain the porting intention and wait for a reply.
If Valve Team should accept the porting, the permission will be added to the header with the name of who has benefited of the license agreement.
That's it. Valve, Epic, Id, are companies. Not evil pits full of demons located at the center of earth (well, maybe id has a closet with brooms and demons, but that's another story!

). Their scope is profict, like every company else in the world. Talk with them is the only method to do things correctly.